The present invention relates to the magnetic resonance imaging techniques. It finds particular application in conjunction with steady state free precession (SSFP) techniques for imaging human patients and will be described with particular reference thereto. It is to be appreciated, however, that the invention may also find application in conjunction with other magnetic resonance applications.
In steady state free precession imaging, a 90.degree. radio frequency pulse is applied concurrently with a slice or slab select gradient along a first axis. A rephasing pulse along the slice select direction follows, commonly concurrently with a primary phase encode gradient along a second or primary phase axis. Next, a dephasing gradient pulse is applied on the read axis. For three dimensional imaging, a secondary phase encode gradient along the slice select axis is applied concurrently with a dephasing gradient pulse on the read axis. A read gradient is applied during the time in which a resultant magnetic resonance echo is predicted. The area of the read gradient from its commencement to the center of echo is substantially the same as the area of the dephasing read gradient, but of the opposite polarity. The sequence is cyclically repeated with the next RF pulse applied immediately following the read gradient to compress the repeat time (TR).
In steady state free precession imaging, a series of phase coherent radio frequency (RF) pulses are applied to excite spins or resonance within a sample. The pulse separation is less than or comparable to the characteristic T.sub.2 and T.sub.1 times for the sample under investigation. As explained in greater detail in Kaiser, et al., "Diffusion and Field Gradient Effects in NMR Fourier Spectroscopy", J. Chem. Phys., Vol. 60, p. 2966, (1974), the successive RF pulses split the magnetization into several components. One of these components represents the magnetization which has been in a transverse state for at least one repeat interval (TR). The algebraic sign of its accumulated spatial encoding is changed by each successive RF pulse.
Unfortunately, difficulties arise when applying SSFP principles in practice. The sequences which cause the magnetization to approach steady state free precession not only derive image intensity or contrast from the free induction decay following each RF pulse, but also from the residual transverse magnetization signals which persist from the previous RF excitations. At many locations within the object being imaged, the primary phase encode gradient, i.e. the gradient which changes amplitude between successive RF pulses effectively destroys coherence of the residual magnetization, hence, its contribution to subsequent primary echo views.
However, the coincidence of the residual magnetization is disturbed very little for locations that lie around the physical center of the phase encoding gradient. Thus, the residual magnetization contribution becomes relatively large adjacent this location.
More specifically, the dephasing read gradient pulse phase encodes the spins of the residual magnetization in the read gradient direction by an offset phase that is proportional to the area of the pulse. The read gradient being of the opposite polarity, cancels this phase encoding and returns it to zero at the center of the read gradient pulse, i.e. where the area of the read gradient pulse matches the area of the opposite polarity dephasing read gradient pulse. Because this is normally the center of the read gradient pulse, the same phase offset but with the opposite polarity is added by the last half of the read gradient pulse. The 90.degree. RF pulse of the next repeat reverses the polarity of the phase offset matching it to the polarity of the next dephasing read gradient pulse. At the end of the read gradient dephasing pulse, the spins have acquired twice the phase offset. The primary read gradient has twice the area of the dephasing read gradient pulse and of the opposite polarity. Accordingly, this phase offset cancels at the end of the read gradient, causing the magnetization to refocus into a residual magnetization gradient echo.
This residual magnetization echo, centered at the end of the read gradient, superimposes residual magnetization data on the primary magnetic resonance view centered. In the next repetition, this residual magnetization component analogously refocuses in the center of the read gradient. Further, an additional residual magnetization component which commences with this cycle refocuses as described above to cause an echo at the end of the read gradient of the next repeat cycle. This process repeats, adding new residual magnetization components as older ones gradually decay in amplitude.
This causes the resulting image to exhibit regions of normal contrast where the coherence is destroyed by the phase encode gradients. Other regions of the image have progressively altered contrast at locations about the physical center of the phase encoding direction. These contrast changes correspond to, respectively, regions of predominantly T.sub.1 contrast and regions where contrast is a function of both T.sub.1 and T.sub.2. Physically, this image contrast artifact appears as a high intensity bright band, commonly referred to as banding.
Two prior art methods are commonly practiced to preserve T.sub.1 contrast In one method, the phasing or spoiling gradients are added after data sampling. The other method is to use small flip angles, rather than the 90.degree. RF pulses. See U.S. Pat. No. 4,707,658, issued Nov. 17, 1987 to Frahm, et al. Both of these techniques reduce the amplitude of the residual magnetization.
Both of these methods, however, encounter difficulties. Applying spoiler gradient pulses after data collection places very large demands on the gradient amplifiers, restricting sequence performance. Further, with very short repeat times and large flip angles, the spoiler gradients may not completely suppress the signal from the residual magnetization. Intensity artifacts still appear in the image. Using smaller flip angles has the undesirable effect of reducing the degree of T.sub.1 contrast relative to 90.degree. flip angles.
Another technique for removing intensity artifacts is shown in U.S. Pat. No. 4,699,148 of Gyngell. A rephasing gradient pulse on the phase encode axis is applied to counteract the effects of the primary phase encoding pulse. Although this method has been successful, it does have certain drawbacks. Specifically, the overall image contrast is a function of both T.sub.1 and T.sub.2. For some applications, T.sub.1 weighted contrast is desired, rendering the Gyngell images less advantageous.
Another method for removing intensity artifacts is described in "Artifacts Due to Residual Magnetization in Three Dimensional MRI", M. L. Wood, et al., Medical Physics, Vol. 50, pp. 825-31 (1988). Wood rotates the intensity bands into the slice direction and then offsets the slice such that it does not coincide with the physical center of the slice select gradient. The described Wood technique successfully removes intensity artifacts from two dimensional Fourier transform SSFP sequences without introducing T.sub.2 contrast. However, the Wood method does not work with three dimensional Fourier transform SSFP sequences. Wood has also proposed that a large static gradient pulse applied after data collection on each axis, in conjunction with a rephasing phase encode pulse on the primary phase axis would eliminate intensity artifacts and T.sub.2 weighted contrast in three dimensional SSFP sequences. In practice, this technique requires prohibitively large static pulses at magnetic field strengths greater than 0.5T to eliminate the T.sub.2 weighted contrast associated with rephasing the phase encode pulse.
As illustrated by U.S. Pat. No. 4,795,978 to Zur, et al., intensity artifacts for three dimensional SSFP sequences can be removed with hardware modifications.
The present invention contemplates a new and improved magnetic resonance technique which overcomes the above referenced problems and others.